The electronic article surveillance (EAS) industry has looked at large to tag devices of a type involving a dipole antenna housed with a diode in a protective envelope of insulative material. In some instances, EAS systems have provided for the transmission of a high frequency signal, such as a 915 megahertz carrier, and of a lower frequency signal, such as modulated 100 kilohertz. Widespread understanding, as evidenced in Pinneo et al. U.S. Pat. No. 4,413,254, is that such device defines a so-called "receptor-reradiator", returning to the receiver of the EAS system, the 915 MHz carrier with content related to the lower frequency transmission and its modulation characteristic. Upon detection in the receiver of received signals inclusive of the modulation characteristic in given repetitive succession, an alarm indication is provided. Generally, detection takes place in a controlled zone, i.e., an exit area of a retail establishment, and output alarm indication is that of a tag device being carried therethrough without authorization (undeactivated).
Subsequent to the Pinneo et al. patent development, the art, particularly through research and development supported by the assignee of the Pinneo et al. patent and this application, has realized substantial analytical evaluation of the activity at hand in EAS dipole and diode tag devices. Thus, in Woolsey et al. patent application Ser. No. 488,077, filed on Apr. 25, 1983, an appreciation flowing from such evaluation is stated, i.e., the need for the establishment of circuit parameters which maximize the reception of the various signals transmitted, the need for establishing an inductive tag device character at the high frequency, where length parameters otherwise dictate, and the need of having a resonant circuit in the tag device at the high frequency.
In addressing such discerned needs, the Woolsey et al. application looks to the addition of inductance at 915 MHz selectively, as by a serpentine inductive path providing same within the length constraint at hand. The Woolsey et al. application thus looks not to the simple dipole/diode combination but to a discernment of specific diversely characterized tag device areas. The device of the Woolsey application thus provides a generally rectangular tag configuration and devotes area to a circuit element, which is inductive at the high frequency and is capacitive up to the lower frequency, and devotes other area to another circuit element, which is inductive at the high frequency, such circuit elements being physically disparate in geometry and arranged in electrical series circuit with the diode. In particular, Woolsey et al. recognize that the sum of the various reactances of the circuit elements and that of the diode should give rise to situations wherein the diode is at the center of a resonant circuit, wherein the net sum of the various reactances at hand across the tag should then be zero and wherein the circuit elements should be addressed generally to different purposes, e.g., that one thereof should be such as to maximize second lower frequency energy receipt and hence voltage applied to the diode.
Apart from the various recognitions of the Woolsey et al. invention, it is the view of the applicants herein that the art has not yet fully realized optimum parameters of tag devices responsive to plural frequency system transmissions.